1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to an apparatus and method for inter-cell interference mitigation in a wireless communication system.
2. Description of the Related Art
Research is being conducted to develop a next generation communication system, also referred to as a 4th Generation (4G) communication system or an International Mobile Telecommunications-Advanced (IMT-Advanced) communication system. Representative examples of such a next generation communication system include a communication system based on an Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard (referred to as an IEEE 802.16m system) and a communication system based on a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Advanced standard (referred to as an LTE Advanced system), both of which are under development. The IEEE 802.16m and the LTE Advanced systems both employ an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme so that a broadband network can be supported in a physical channel.
In a wireless communication system employing the OFDM/OFDMA scheme, orthogonal radio resources are allocated by a Base Station (BS) to one or more Mobile Stations (MSs) within a cell coverage area of the BS. Therefore, interference between the MSs within the BS's cell coverage area is not significant. However, if the same radio resource is used in a neighboring cell, an MS located at or adjacent to an edge of the BS's cell may experience interference. Such interference is referred to as inter-cell inference.
FIG. 1 is a schematic diagram for explaining the generation of inter-cell inference in a conventional wireless communication system.
Referring to FIG. 1, an MS 130 receives downlink data by decoding a signal received using a radio resource allocated by a serving BS 110. In this case, if the radio resource allocated by the serving BS 110 to the MS 130 is identical to a radio resource used by an interfering BS 120, the MS 130 receives both a downlink signal transmitted by the BS 110 and a downlink signal transmitted by the BS 120. Accordingly, the MS 130 performs decoding on a mixed signal of the two downlink signals and thus data reception may not be properly achieved. That is, the downlink signal transmitted by the BS 120 acts as interference to the MS 130.
As described above, in a wireless communication system using the OFDM/OFDMA scheme such as the IEEE 802.16m or the LTE Advanced systems, inter-cell interference results in system performance deterioration due to DownLink (DL) or UpLink (UL) signals transmitted from different cells that are using the same frequency resource or time resource.
In addition to the OFDM/OFDMA scheme, the IEEE 802.16m and the LTE Advanced systems both employ a Multiple-Input Multiple-Output (MIMO) scheme. In comparison with a wireless communication system using a single antenna, a wireless communication system using multiple antennas in both transmitting and receiving ends may increase channel transmission capacity in proportion to the number of antennas without additionally allocating frequencies or Transmit (Tx) power. Another benefit of the MIMO scheme is that it allows for mitigation of inter-cell interference.
To mitigate inter-cell interference, a wireless communication system using the MIMO scheme, such as the IEEE 802.16m or the LTE Advanced systems, uses a precoding matrix for inter-cell interference mitigation. That is, in DL communication, a BS multiplies a Tx signal by the precoding matrix in order to cancel interference it is causing to a neighbor cell and maximize downlink performance within its cell. The precoding matrix used for interference mitigation is one of a plurality of precoding matrices included in a codebook, wherein each of the precoding matrices can be identified by a Precoding Matrix Index (PMI). The particular codebook used by the BS depends on the standard on which the operation of the BS is based.
A PMI coordination scheme is employed to mitigate inter-cell interference using the precoding matrix. In operation, an MS located near an edge of a cell measures the DL channel of the serving BS and any dominant interfering BSs. For convenience in explanation, it will be assumed hereafter that there is one interfering BS. The MS then determines an appropriate PMI for its serving BS and the interfering BS. When determining the PMI, the MS may determine one or both of a preferred and an unwanted PMI. Once the PMIs for the serving and interfering BS are determined, the MS feeds back the PMI for the serving BS and interfering BS to its serving BS. The serving BS utilizes the PMI intended for it and forwards the PMI for the interfering BS to the interfering BS via a backhaul. The interfering BS adjust its DL transmission based on the PMI received from the serving BS. Thereby, inference to the MS caused by the interfering BS is reduced.
However, DL channel conditions vary over time and location. As a result, there is a problem in that by the time the interfering BS applies the PMI, the DL channel conditions may have changed too much for the PMI scheme described above to be effective. In other words, the PMI scheme described above suffers from a PMI aging problem when there is too much delay between the MS measuring the DL channel and the interfering BS adjusting its transmission. The delay is primarily caused by feedback delay and backhaul delay. Feedback delay corresponds to the amount of time taken for the MS to select a PMI from all of the PMIs in the codebook and then forward the selected PMI to the serving BS. Backhaul delay corresponds to the amount of time taken to communicate the PMI received from the MS by the serving BS to the interfering BS over a backhaul connection. Furthermore, the amount of time a PMI can age before it is unreliable is inversely related to a rate of mobility of the MS. Accordingly, in practical applications the PMI scheme described above is limited to use with MSs having a low rate of mobility.
Therefore, a need exists for an apparatus and method for mitigating inter-cell interference that does not suffer the PMI aging problem and that can be used with MSs having a high rate of mobility.